1. Field of Invention
The present invention relates to a method for fabricating polysilicon film and a method for fabricating thin film transistors using the described method, and particularly to a method using heterogeneous growth to fabricate polysilicon film and a method for fabricating thin film transistors using the described method.
2. Description of the Related Art
Display devices are interface between people and information. Presently, flat displays are the new trend on the development of technology. Flat displays mainly include organic electro-luminescence display (OELD), plasma display panel (PDP), liquid crystal display (LCD) and light emitting diode (LED) etc.
Thin film transistors (TFT) can be used as driving elements of the above-mentioned displays. In general, TFTs can be distinguished into amorphous silicon TFTs (a-TFT) and low-temperature polysilicon thin film transistors (LTPS-TFT) according to the materials of the channel layer. Wherein, an LTPS-TFT is formed by a technique that is better than the traditional technique of forming amorphous TFTs. Because the electronic mobility of the LTPS-TFTs can be reached up to or over 200 cm2/V-sec, the area of the TFTs can be decreased so as to conform to the high aperture requirement of displays. As a result, the brightness of displays can be increased and the entirely power consumption can be decreased. In addition, due to the improvement of the electronic mobility, partial driving circuits of displays can be formed on the substrate, so-called chip on glass (COG), during the manufacturing process so as to significantly reduce the fabricating cost of display panels.
It should be noted that the channel layer of LTPS TFT is composed of polysilicon film formed by excimer laser annealing (ELA). Therefore, heterogeneous growth method is usually used for forming polysilicon film with better quality, less defect and larger grain.
FIG. 1A to FIG. 1E are cross-sectional diagrams of the procedures of using conventional heterogeneous growth method to form polysilicon film. Referring to FIG. 1A, a substrate 100 is provided first, and sunken patterns 110 are formed on the substrate 100. Wherein, the method for fabricating the sunken patterns 110 may be the conventional photo-lithography and etching process. Then, referring to FIG. 1B, an amorphous silicon film 120 is deposited in the sunken patterns 110. Referring to FIG. 1C, the amorphous silicon film 120 (as shown in FIG. 1B) in the sunken patterns 110 is transferred into a polysilicon film 140 by performing an ELA process 130. Referring to FIG. 1D, another amorphous silicon film 150 is deposited on the substrate 100. Later, referring to FIG. 1E, the amorphous silicon film 150 (as shown in FIG. 1D) is transferred into a polysilicon film 170 by performing another ELA process 160.
In more detail, the polysilicon film 140 formed in the sunken pattern 110 is used as a seed layer. When the ELA process 130 (as shown in FIG. 1E) is performed, because of the difference of the melting points between the amorphous silicon film 150 and the silicon film 140 in the sunken patterns 110, the polysilicon film 140 is used as the seed of the amorphous silicon film 150 for performing heterogeneous growth.
However, the above-mentioned method for fabricating the polysilicon film 170 has to first form sunken patterns 110 on the substrate 100 by performing photo-lithography process, and the polysilicon film 140 used as a seed layer has to be formed in the sunken patterns 110. In other words, the above-mentioned method for fabricating the polysilicon film 170 has to perform the ELA process twice. As a result, it increases the process steps and time.